This is the first Voigtlander C lens in my possession after going through a few hundred in the past 10 years Not sure how many are out there. From what I can find, it is supposed to shorten the distance by about 1/4 by replacing the back group and also increase the illumination in the proportion 9:16. What does this mean actually? The lens was from a studio camera for portrait in the old days.

Tuant